Numerous homopolymers, copolymers, terpolymers, graft polymers and their mixtures with each other and also with naturally occurring resins or waxes have been proposed for use as polymeric adhesives. The demands of modern technology, however, are for ever higher requirements of adhesion and cohesion. Adhesion should take effect immediately on application, if possible. In addition, these properties must not be lost by ageing.
Polymer adhesives of this kind are frequently used as pressure-sensitive adhesives on a great variety of carriers, especially on self-adhesive tags, labels, adhesive tapes, decorative sheets and other types of sheets, and for floor coverings.
Pressure-sensitive adhesives should combine the properties of permanent surface adhesion and cohesion to the greatest possible extent. They are intended to adhere to a wide variety of substrates when subjected to gentle pressure, that is to say, without being pressed on by rubbing and without being moistened with solvents, and to peel off the substrate again without leaving traces of adhesive behind. Transfer of the adhesive from the carrier to the substrate on separation is undesirable.
Pressure-sensitive adhesives previously frequently used commercially are, for example, block copolymers of styrene with isoprene or butadiene and ethylene/vinyl acetate copolymers, which are used as mixtures with low-molecular-weight resins, waxes, plasticizers or solvents.
These pressure-sensitive adhesives can be employed in the industrially advantageous melt-application process only when using polymerization products of low-molecular weight and large additions of low-molecular weight substances, such as those mentioned above. In many cases, however, cohesion and resistance to ageing are impaired thereby. In addition, the adhesives obtained in this manner are only poorly suited for application to plastic material, especially plastic sheets, that have been plasticized with so-called "monomer" plasticizers, for example, phthalate and/or phosphate plasticizers. Examples of such plastic materials are homopolymerization products and copolymerization products of ethylene, propylene, unsaturated esters and/or vinyl chloride.
During ageing, or even more rapidly when a plastic article, especially a sheet, so coated is subjected to an elevated ambient temperature, the plasticizer migrates appreciably into the adhesive layer. The properties of the adhesive are adversely affected as a result: the cohesion decreases sharply, and the adhesive becomes greasy, tends to migrate into porous substrates and to leave behind substantial amounts of adhesive when the plastic article is detached from the substrate again. In the case of plasticized plastic sheets, the migration of plasticizer additionally leads to considerable shrinkage of the sheet. This is apparent, for example, from unsightly dirty edges around the stuck-on sheet. The problem of the sheet shrinking is more especially apparent in the use of decorative sheets of soft polyvinyl chloride (soft PVC), such as are often used both commercially and in the home. In this case, shrinkage of the sheet results in ugly gaps at the edges and butt joints of areas of sheet affixed next to each other.
U.S. Pat. No. 3,911,053 describes ethylene/vinyl ester graft copolymers and their use as hot melt pressure-sensitive adhesives. These graft copolymers, which are particularly suitable, for example, for self-adhesive paper labels, can also be applied to plasticized plastic articles, e.g. plastic sheets. To avoid the above-mentioned disadvantages, these graft copolymerization products have to be mixed additionally with plasticizers, so that further migration of the plasticizer contained in the plastic material into the layer of adhesive is prevented, and, moreover, their K-values must be very high so that the plasticizer-containing adhesive mixtures do not become greasy.
Bonds produced with such adhesives may become loose in the course of time, and, in the case of substrates that adhere well, a large part of the adhesive frequently remains behind on the substrate, subsequently necessitating expensive cleaning operations. The same occurs when detaching the affixed article, for example, a plastic sheet. The fact that the adhesion of the adhesive to the carrier is too slight necessitates an expensive pre-treatment of the carrier to improve adhesion. This is usually effected by applying a priming coat, or by physical or chemical treatment, for example, with electrical discharges (corona pre-treatment), by flame-treatment or by mechanical roughening. The pre-treatment of the surface is frequently necessary even with carriers that do not contain plasticizers; for example, textiles, plastics or metals, in order to prevent the pressure-sensitive adhesives of U.S. Pat. No. 3,911,053 from loosening.
Particularly when using relatively large sheets coated with pressure-sensitive adhesive, for example, decorative sheets of plastic or metal, it is easy inadvertently to stick the reverse of the sheet material to itself, that is, with the films of adhesive together. If the pieces of sheet material are then pulled apart, it is generally impossible to separate the two films of adhesive again when the sheets have not been pre-treated, and one of the two films detaches itself from the surface of the sheet on account of too low an adhesion. The adhesive sheet thus becomes useless.